Method for improving formability of hexagonal close packed metals

ABSTRACT

A method of improving the formability of a strip of sheet magnesium or other hexagonal closely packed metal is comprised of repeatedly bending, straightening and reverse bending the strip along its entire continuous length to induce plastic deformation and twinning. Twinning reduces and redistributes the grain size of the sheet material to make the sheet material more formable during subsequent metal forming operations. In practice, a coil of sheet magnesium strip is mounted on a spindle. The end of the strip is unwound from the coil and threaded through a series of offset rollers. The end of the strip is connected to a take-up spindle located down stream of the offset rollers. The strip is unwound from the storage spindle and progressively and continuously bent and reverse bent as it traverses around the offset rollers and is recoiled onto a take-up spindle.

FIELD OF THE INVENTION

The present invention relates to a method for improving the formability of a sheet of magnesium or other metal having hexagonally close packed crystal structure.

BACKGROUND OF THE INVENTION

It is known in the auto industry to manufacture vehicle body panels by stamping or otherwise forming sheet metal panels. Such manufacturing processes entail the forming of the sheet metal panels between opposing dies so that the sheet metal experiences plastic deformation and the shape of the dies is permanently imparted to the metal to define the shape of the manufactured article.

It is known to manufacture automotive panels using magnesium sheet, particularly magnesium alloy AZ31.

It is known that the efficiency and the quality obtained in metal forming processes is dependent of the degree of ease or difficulty experienced in forming the sheet metal panel. Accordingly it is desirable to provide sheet metal panels having improved and optimal formability characteristics. One characteristic that influences the formability of sheet metal panels is the grain structure of the metal, and in particular the grain size and the distribution of grain size within the microstructure of the metal.

With respect to metals having a hexagonal close packed microstructure, such as magnesium, it is known that plastic deformation of the metal can change the lattice structure by a phenomenon known as deformation by twinning. In twinning, the atoms of the hexagonal close packed structure are moved a fraction of an interatomic space relative one another, leading to a re-arrangement of the lattice structure.

SUMMARY OF THE INVENTION

According to the invention a method of improving the formability of a strip of sheet magnesium or other hexagonal closely packed metal is comprised of repeatedly bending, straightening and reverse bending the strip along its entire continuous length to induce plastic deformation and twinning. Twinning reduces and redistributes the grain size of the sheet material and thereby makes the sheet material more formable during subsequent metal forming operations. In practice, a coil of sheet magnesium strip is mounted on a spindle so that an end of the strip can be unwound from the coil. The end of the strip is then threaded through a series of offset rollers. The end of the strip is connected to a take-up spindle located down stream from the offset rollers. The strip is unwound from the storage spindle and progressively and continuously bent and reverse bent as it traverses around the offset rollers and is recoiled onto the take-up spindle.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, which:

FIG. 1 is an elevation view showing apparatus including a storage spindle having a strip of sheet metal coiled thereon, a series of offset rollers through which the strip of sheet metal passes, and a take-up spindle for recoiling the strip.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The following description of the certain exemplary embodiment's embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

Referring to FIG. 1 it is seen that a coil of sheet metal strip 10 is supported on a spindle 12. The sheet metal strip 10 is a metal having a close packed hexagonal microstructure, such as magnesium, zinc, titanium or zirconium. A series of rollers 14, 16, 18, 20, and 22 are located at a spaced distance from one another and from the spindle 12. In particular as seen in FIG. 1, roller 14 is mounted on a shaft 26, and roller 16 is mounted on a shaft 28. The roller 16 is mounted both vertically and laterally spaced from the roller 14. Likewise the rollers 18, 20, and 22 are likewise mounted on their respective shafts 30, 32, and 34 in lateral and vertical spaced relation. A take-up spindle 40 is mounted at a distance from the roller 22.

As seen in FIG. 1 the sheet metal 10 is threaded around the rollers 14, 16, 18, 20, and 22 and then connected to the take-up spindle 40.

The take-up spindle 40 is connected to a drive mechanism, not shown, which will rotate the take-up spindle 40 in the clockwise direction so that the sheet metal panel is progressively pulled through the tortuous path defined by the rollers 14, 16, 18, 20, and 22, and then recoiled onto the take-up spindle 40. It may be desirable to connect one or more of the rollers 14, 16, 18, 20, and 22, as well as the spindle 12 to a drive mechanism in order to facilitate the operation of the apparatus.

It will be understood and appreciated that as the sheet metal strip 10 passes onto the roller 14 at the point designated “A”, the sheet metal strip will be bent to an extent that is determined by the radius of the roller. Then as the sheet metal strip 10 exits its passage around the roller 14 at the point designated “B”, the sheet metal strip 10 will be unbent and resume a straightened condition. Thereafter, upon reaching the roller 16 at the point designated “C”, the sheet metal strip 10 is bent in the opposite direction from the bending that occurred on roller 14 at point “A”. Thereafter, as the sheet metal coil reaches the end of its passage around roller 16 at point “D”, the sheet metal strip 10 will be straightened again. Thus it is seen that as the sheet metal traverses the rollers 14, 16, 18, 20 and 22, the sheet metal strip 10 will be first bent in one direction, then returned to a straightened condition, and then reversely bent in the opposite direction. This bending, straightening and reverse bending of the sheet metal strip 10 occurs repeatedly and continuously along the length of the strip 10 as the sheet metal strip 10 is uncoiled from the spindle 12 and then recoiled onto the take-up spindle 40.

In practice, we have observed that the afore described progressive and continuous bending, straightening and reverse bending of the sheet metal strip induces mechanical twinning into the sheet metal strip 10 and provides a beneficial effect on the grain size and grain size distribution of the sheet metal material that will make the strip more formable during subsequent metal forming operations such as stamping, or flanging at room temperature or super plastic forming or quick plastic forming at elevated temperatures.

The extent of improvement in the formability of the sheet metal material may be varied by controlling several variables in the apparatus and method. For example, the afore described bending and reverse bending process can be performed with the sheet metal strip at room temperature. Alternatively, the process can be performed with a sheet metal panel heated to a temperature in the range of 50 degrees centigrade to 450 degrees centigrade.

Although the drawings illustrate the invention as including a strip of coiled metal and a take-up spindle, the invention may also be practiced by passing a flat blank of sheet material though a series of offset rollers, in which case the sheet will be rolled to its flat condition after being bent and reverse bent by the offset rollers.

Furthermore, it may be advantageous, after the sheet metal has been recoiled onto the take-up spindle 40, to re-crystallize the grain structure of the metal by heating the sheet metal to a temperature of 350 degrees centigrade for approximately 15 minutes, or other suitable combinations of time and temperature.

In addition it will be appreciated that the diameter of the rollers 14, 16, 18, 20 and 22 will determine the how sharply the strip will be bent. By reducing the diameter of the roller, the strip will be bent more severely, while increasing the diameter of the rollers will decrease the severity of the bend. The rollers may be of the same diameter, or the rollers may be of differing diameter, and arranged, for example, with the diameters in ascending or descending order. If the rollers are alternated with first a small roller and then a larger roller, then a smaller roller, etc, the strip will be bent more severely in the one direction than in the reverse direction. The number of times that the strip is subjected to the bending and reverse bending will be dependent on the number of rollers that are used. 

1. A method of improving the formability of a strip of metal of close packed hexagonal microstructure comprising repeatedly bending and then reverse bending the sheet along its entire continuous length to induce plastic deformation and twinning.
 2. The method of claim in 1 which the repeated bending and reverse bending of the strip is obtained by passing the strip through a series of offset rollers so the strip is repeatedly and continuously bent in one direction and then in the other direction.
 3. The method of claim 2 in which the strip of metal is magnesium.
 4. The method of claim 3 in which the metal is magnesium AZ31.
 5. The method of claim 1 Including the further step of, subsequent to the bending and reverse bending, heating the strip of material in the range between 50 degrees Centigrade and 450 degrees Centigrade to re-crystallize the grain structure of the strip.
 6. The method of claim 2 in which the diameter of the rollers is chosen to provide the desired severity of bending and unbending.
 7. The method of claim 2 in which the number of rollers is chosen to provide the desired number of times that the strip is bent and reverse bent.
 8. A method of improving the formability of magnesium sheet comprising: passing the sheet through series of offset rollers to effect repeated bending and unbending the sheet along its entire continuous length to induce plastic deformation and twinning whereby the grain size of the sheet is reduced for improved subsequent formability.
 9. The method of claim 8 in which the number of rollers is selected in accord with the number of bendings and reverse bending to which the sheet will be subjected and the diameter of the rollers is selected in accord with the severity of the bendings and reverse bending to which the sheet will be subjected.
 10. The method of claim 9 in which the sheet is magnesium AZ321.
 11. The method of claim 10 including the further step of, subsequent to the bending and reverse bending, heating the strip of material in the range between 50 degrees Centigrade and 450 degrees Centigrade to re-crystallize the grain structure of the strip.
 12. A method of improving the formability of a sheet of magnesium AZ31 comprising: mounting a coil of sheet magnesium on a spindle so that an end of the sheet can be unwound from the coil, threading the end of the sheet through a series of offset rollers, and recoiling the end of the sheet on a take-up spindle located downstream from the offset rollers so that sheet of magnesium is repeatedly bent and unbent along its entire continuous length by the offset rollers during uncoiling from the storage spindle and recoiling onto the take-up spindle, whereby to introduce continuous twinning into the sheet of magnesium and thereby modify the grain structure of the sheet material to improve its formability in subsequent forming of the magnesium sheet.
 13. The method of claim 12 in which the diameter of the rollers is chosen to provide the desired severity of bending and unbending.
 14. The method of claim 12 in which the number of rollers is chosen to provide the desired number of times that the strip is bent and reverse bent.
 15. The method of claim 12 in which the metal is magnesium AZ31.
 16. The method of claim 12 including the further step of, subsequent to the bending and reverse bending, heating the strip of material in the range between 50 degrees Centigrade and 450 degrees Centigrade to re-crystallize the grain structure of the strip. 